This applications is a national stage of PCT/DE98/01237 filed May 2, 1998 and based upon German national application 197 19 195.9 of May 9, 1997 under the International Convention.
The invention relates to a cutting insert for machining operations, consisting of a hard metal, cermet or ceramic substrate body with a multilayer coating.
Substrate bodies coated with a hard materials, optionally also coated with multiple layers, are known to the state of the art. As a rule the hard-material coating serves for creating a wear-resistant surface layer, which is combined with tough substrate bodies mechanically capable to bear high loads. According to the state of the art, two different methods are used, so-called chemical vapor deposition (CVD) or the physical vapor deposition (PVD). The usual protective layers consist for instance of TiC, TiN and/or aluminum oxide. It has also been proposed to apply multiple layer coatings with the layer sequence TiN, Ti(C,N), Ti(C,N), TiN on a substrate body mit any desired C:N mixing ratios.
From DE 195 30 517 also metal carbonitride hard materials on a substrate body are known, wherein the metal of the metal carbonitride layer contains two or more of the elements of the group Ti, Zr, Hf, V, Nb, Ta, Cr, Mo and W. The layer is described particularly as a quaternary layer of (Ti,Zr)(C,N) obtained through CVD. As a special embodiment example for an indexable insert, a coating consisting of an approximately 1 xcexcm thick TiN layer, a 8 xcexcm thick (Ti,Zr)(C,N) layer and a last layer with a thickness between 3 and 5 xcexcm of Al2O3 was deposited on a hard metal substrate body.
The already mentioned PVD coatings, which can be already produced starting from approximately 400xc2x0 C., are used especially for temperature-sensitive substrate bodies, particularly for the coating of rapid machining steel, which during coating should not be subjected to high temperatures.
The references WO 92/05296 or U.S. Pat. No. 5,143,488 with a similar content indicate in addition that TiN layers deposited through the CVD process show tensile strains, while TiN coatings applied through the PVD process show compressive strains which do not lead to fracture as is the case with tensile strains. In order to improve the fracture resistance, these references propose to combine one or more CVD-layers with one or more layers deposited through PVD. As materials for the inner layer deposited by CVD, nitrides of titanium, hafnium and/or zirconium are proposed, and for the layers deposited by PVD nitrides and carbonitrides of the mentioned metals are proposed. From the point of view of process technology, there is a disadvantage in that two successive, relatively expensive, different coatings (CVD, PVD) have to be applied by means of different apparatus. Furthermore it has been found that the multilayer coating consisting of inner coatings deposited through CVD and at least one outer layer deposited through PVD tend to detach themselves from each other in case of thicker layers.
It is therefore an object of the present invention to avoid the aforementioned drawbacks. The cutting insert should have a high fracture resistance (bending strength). The process required for its manufacture should be cost-effective and performed with the minimum possible equipment expenditure.
A cutting insert for machining operations can consist of a cermet or ceramic substrate body with a multilayer coating, whose outer layer (cover layer) consists of a single-phase or multiphase layer of carbides, nitrides or carbonitrides on a Zr or Hf basis, which was applied by means of CVD and presents inner compressive strains. Its underlying layers applied also by CVD have exclusively inner tensile strains, whereby at least one or the sole underlying layer consists of TiN, TiC and/or Ti (C,N).
The multilayer coating on a hard metal, cermet or ceramic substrate body has an outer layer (cover layer) consisting of a multiple phase layer of carbides, nitrides or carbonitrides on a zirconium or hafnium basis and which has inner compressive strain. The underlying layer or layers, without exception, have tensile stresses, whereby at least one or the sole underlying layer consists of titanium nitride, titanium carbide and/or titanium carbonitride. All layers are applied by CVD, whereby also such embodiments are included wherein the cover layer can consist of several individual layers of the same or of a different material composition. It is essential for the present invention that the cover layer contain zirconium or hafnium which can be in the form of a carbide, nitride or carbonitride and can be combined with an underlying layer of a titanium carbide, nitride and/or carbonitride. Preferably the compressive strains range between xe2x88x92500 to xe2x88x922500 MPa (compressive strains are defined with negative values in contrast to tensile strains which are defined with positive values).
As a single-phase cover layer a carbide, nitride or carbonitride of zirconium or hafnium can be selected. Alternatively thereto, there are also single-phase cover layers consisting of a carbide, nitride or carbonitride of the form (M1,M2) (CxN1xe2x88x92X) with M1=Zr or Hf and M2=Ti, Hf, Nb, whereby 0xe2x89xa6xxe2x89xa61. In the latter case they are also defined as so-called quaternary multimetal carbonitrides.
A preferred two-phase outer layer consists of a carbide, nitride or carbonitride of zirconium or hafnium and ZrO2. ZrO2 can be present in monocline, tetragonal and/or cubic modification. Particularly the outer layer can contain a composition of the form Me(CxN1xe2x88x92x) with Me=Zr or Hf and 0.3xe2x89xa6xxe2x89xa67. If the cover layer is a single phase layer, then it differs from the two-phase layer in that it has a uniform structure and lattice constant, which includes particularly tertiary and quaternary carbides, nitrides or carbonitrides, wherein zirconium or hafnium must be present either alone or together for at least 50% by atom. The respective metal atoms then distribute themselves statistically on a sublattice, while their metalloids, carbon and nitrogen are also randomly distributed on the other sublattice.
In a two-phase layer, which consists particularly of a carbide nitride or carbonitride of zirconium or hafnium, in addition also containing ZrO2, two phases, which are different as to their crystalline structure and lattice, coexist next to each other. The second phase ZrO2 amounts preferably to a proportion between 15 and 60% by mass.
According to a further embodiment of the invention, between the cover layer and at least one layer close to the substrate body, one of the lower layers can consist of Al2O3.
Preferred thicknesses of the cover layer range between 2 xcexcm and a maximum of 10 xcexcm, whereby further preferably the total thickness of the inner layers subject to tensile strains and the outer layers subject to compressive strains ranges between 5 xcexcm and 25 xcexcm, particularly between 10 and 20 xcexcm.
Preferably a first TiN-layer with a thickness up to 2 xcexcm and one or more TiCN-layers of up to 5 xcexcm are deposited on the substrate body, on which a single-phase or two-phase cover layer with a thickness of up to 5 xcexcm has been deposited.
For the production of a cutting insert of the aforementioned kind the coating is performed in an uninterrupted CVD process at temperatures between 900xc2x0 C. and 1100xc2x0 C. through respective changes of the gas composition.
The compressive strain of an outer layer of a single-phase multimetal carbonitride can be increased particularly due to the fact that after the CVD deposition follows a heat treatment at a temperature higher than the substrate body temperature during the last deposition. The respective temperatures lie between 95xc2x0 C. and below the eutectic temperature of the substrate body material. The heat treatment lasts at least for 15 minutes, so that a spinodal segregation into a titanium-rich and a zirconium-rich phase takes place.